The invention pertains to a photovoltaic device of the thin film type comprising a supporting substrate and an active layer. The invention further pertains to a process for producing a photovoltaic device comprising a supporting substrate and an active layer of an amorphous material.
Photovoltaic devices are commonly used to convert light energy into electrical energy. Photovoltaic devices contain an active layer which consists of a light absorbing material which generates charge carriers upon light exposure. An active layer which is currently common in photovoltaic devices is silicon (Si). However, a variety of materials can be encountered like for example gallium arsenide (GaAs), cadmium telluride (CdTe) or copper indium gallium diselenide (CIGS). The charges, which are generated in the active layer, are separated to conductive contacts that will transmit electricity. Due to the thin and brittle nature of the active layer it is usually protected from external influences by a transparent cover plate e.g. made of glass. It is known from prior art that both the active layer and the cover plate reflect a part of the light incident to the photovoltaic device. Especially the high refractive index of the active layer causes large reflection losses which can—in the case of silicon—be up to 30% of the incident light. Since the reflected light can not be converted into electrical energy these reflection losses cause a large reduction in the efficiency of a photovoltaic device.
In order to reduce these reflection losses, an anti reflection coating can be applied on top of the light absorbing material or so called active layer. An anti reflection coating consists of a single quarter-wave layer of a transparant material with a refractive index which is between the refractive index of the active layer and the cover plate. Although this theoretically gives zero reflectance at the center wavelength and decreased reflectance for wavelengths in a broad band around the center, the processing and material costs of these layers are relativly high. Also the processing techniques to create the coatings (e.g. chemical vapour deposition) are very complex, elaborate and time consuming.
It is known from the art that a surface structure on the light receiving side of the cover layer can be used to reduce the reflection losses of the the cover plate. Here, V-shaped (G. A. Landis, 21st IEEE photovoltaic specialist conference, 1304-1307 (1990)) or pyramidal structures as disclosed in WO 03/046617 are applied to the light receiving side of a cover plate made of glass to reduce the reflection losses of said plate and hence increase its transmission. CH 693 771 A5 discloses cone structures and pyramidal structures with triangular surfaces applied to both sides of the cover layer to reduce the reflection losses of said plate and hence increase its transmission. The structures are applied on both surfaces of the cover layer to prevent negative consequences of upside down installation of the cover layer. The structures can be applied to the glass plate via for example casting or pressing. However, when using the plate as a cover plate of a photovoltaic device the maximum efficiency of said device can only be increased by 6%, which is a reduction of approximatly 30% of the reflection losses, according to a model study (U. Blieske et all, 3rd World Conference on Photovoltaic Energy Conversion, 188-191 (2003)). In practice the results are even less and only 3% can be obtained. Although the structures reduce some of the reflection losses of the active layer, it reduces predomenantly the reflection losses of the cover plate. Hence the total reduction in reflection losses, and increase in efficiency of the photovoltaic device, is low.
A better method to reduce the reflection losses of the active layer via texturing the light receiving surface side of the cover plate is given in non-published European Patent Application EP07 021 458. Here an array of surface relief structures is applied to the cover plate that is in optical contact with the active layer. A single structure of said array is characterized by a base and a single apex and is connected by at least three n-polygonal surfaces where n is equal to 4 or higher. This specific design reduces not only the reflection losses of the glass cover but also to a great extent the reflection losses of the active layer underneath the cover plate. Although this method gives a cost-effective and efficient anti-reflection coating, the relief structures, located at the outside of the photovoltaic device, are sensitive to fouling.
Another method to reduce the reflection losses is to structure the surface of the active layer. This can be done by direct structuring of the material itself. Several processes, as for example sandblasting, milling, anionic oxidations, sputter etching, are known to create these structures. U.S. Pat. No. 4,626,613 discloses a laser assisted process for direct structuring of the active layer. U.S. Pat. No. 5,702,538 discloses photolithographic texturing for direct structuring of the active layer. JP 10163513 A discloses direct etching of the active layer with an etchant. By structuring the active layer, with pyramids, V-shapes, cubical shapes or random structures, a reduction in the reflection losses at active layer is obtained by multiple reflection at the surface offering the light a greater opportunity to enter the panel. Light falling within the grooved area, if it is not initially absorbed by the active layer, will be reflected onto another surface of the groove thereby increasing absorbtion. This effect reduces the reflection losses at the surface of the active layer and is therfore often referred to as an anti-reflection effect. Secondly, the structures may in some cases partially trap the light which is not absorbed by the active layer and reflected by surface of the substrate. As a result the chance of light absorption by the active layer is increased. Although structuring of the active layer can significantly improve the efficiency of a photovoltaic cell, production methods are very complicated, toxic and extremly expensive.
An alternative to direct structuring of the active layer is surface structuring of the substrate i.e. the cover layer on which the active layer is subsequently deposited. This method can be employed for thin film solar cells which use e.g. an active layer such as amorphous silicon. In said case the solar cells are manufactured by applying respectively the front electrode, active layer and back electrode on top of the transparent cover plate. In order to reduce the reflection losses of the active layer the front electrode is randomly textured via wet etching prior to depositing of the active layer.
Instead of a randomly textured surface, surface structuring of the cover layer can also be carried out such that the cover layer receives a defined and repeating surface structure on which the active layer is subsequently deposited. DE 4 201 126 pertains a photovoltaic device with a semiconductor thin film component for electric energy conversion. The active layer i.e. the semiconductor thin film component may be amorphous Silicon. The cover plate of the device has on its rear surface, i.e. the surface on which the active layer is deposited, a defined and repeating surface structure in the shape of a sawtooth formation thus leading to a sawtooth formation of the active layer. The effect of the sawtooth formation is a multiple internal reflection of unabsorbed light.
From the above it can be concluded that several techniques exist to reduce the reflection losses of a photovoltaic device. These techniques comprise structuring of the active layer or applying an anti-reflection coating on top of said layer. They are, however, complex and expensive. Alternatively, an additional transparent structured layer can be applied to the active layer or the cover plate. Although these techniques are cost effective, their performance is less efficient.